One of the known applications for the invention relates to devices designed to correct and hold a set of vertebrae in a determined position. In this application, such a device comprises an intervertebral connection element such as a plate or a rod suitable for being held in position by bone anchor elements such as screws implanted in the vertebrae. Each bone anchor element is provided with a head, and a threaded shank for mounting the plate which has through holes. The plate is engaged on the threaded shank via a through hole and the portion of the shank that projects from the plate is used for receiving a clamping nut.
Given the anatomy of the vertebrae, it is necessary to allow for an adaptable relative angle to exist between the anchor screw and the intervertebral connection element. In order to allow for such an adaptable relative angle, it is known to fit anchor screws with a spherical joint for receiving the intervertebral connection element. Such a spherical joint is arranged between the anchor screw and the threaded shank in order to enable the threaded shank receiving the intervertebral connection element to take up orientations in a plurality of different directions.
Although an anchor screw of the type having a spherical joint makes it possible to achieve an adaptable relative angle between said screw and the intervertebral connection element, it must be considered that making a spherical joint on an anchor screw contributes to weakening it by including a moving interface that can even lead to the intervertebral connection element breaking. Furthermore, it has been found that even in the presence of an anchor screw of the type having a spherical joint, the threaded shank does not always extend to project perpendicularly relative to the intervertebral connection element, given the anatomy of the vertebrae and the zone in which the anchor screw is implanted. It follows that the nut for screwing onto the threaded shank can no longer be tightened properly, insofar as the intervertebral connection element does not extend perpendicularly to the threaded shank, such that the intervertebral connection element is subjected to inappropriate stresses and cannot be completely prevented from moving, at least not with appropriate clamping force.